1. Field of Invention
The present invention relates to a fabrication method for a self-aligned contact opening. More particularly, the present invention relates to a method for improving the self aligned effect of the self-aligned contact opening using polysilicon in order to reduce loss of a cap layer above a word line and a spacer on a sidewall of the word line.
2. Description of Related Art
In the deep sub-micron integrated circuit (IC) process, an opening with smaller size is often formed using a self-aligned contact process. Since the self-aligned contact process indirectly increases the process window of the photolithography, the size of the chip is reduced by using the self-aligned contact process.
In the conventional self-aligned contact process, a nitride cap layer is formed on a word line, with a nitride spacer forming on a sidewall of the word line. A silicon oxide layer is then formed to cover the cap layer and the spacer before coating a photoresist layer on the silicon oxide layer. After the photoresist layer is patterned using photolithography, the silicon oxide layer is etched, using a silicon oxide etching recipe with high etching selectivity for silicon oxide to silicon nitride and the photoresist layer serving as an etching mask. During the etching process, the silicon nitride cap layer above the word line and the silicon nitride spacer on the sidewall of the word line may serve as a secondary mask, so that a self-aligned contact opening is formed in the silicon oxide layer. The self-aligned contact opening is then filled with tungsten or polysilicon to form a plug.
However, some oxygen atoms may be released from the silicon oxide layer into the etching environment when the silicon oxide layer is etched using the conventional silicon oxide etching solution. As the amount of oxygen atoms in the etching solution increases, the degree to which the silicon nitride cap layer and the silicon nitride spacer are removed also increases. As a result, the etching selectivity for silicon oxide to silicon nitride is reduced, causing a more serious parasitic capacitance effect between the word line and the plug, thus the transmission rate of the word line and its adjacent plug are also affected. In an even more serious case, a shortcircuit occurs between the plug and its adjacent word line. With the trend to continuously reduce size of the device, the bottom width of the silicon nitride spacer on the sidewall of the word line is also reduced according to the design rule. This makes performance of the self-aligned contact process even more difficult.